1. Field of the Invention
The present invention relates to a flow meter arrangement, especially one suitable for the measurement of respiratory gas flow associated with the breathing of a patient.
2. Description of the Prior Art
In hospitals, during for example intensive care or surgical procedures, mechanical breathing aids are used to assist or control the breathing of a patient. It is important to be able to measure gas flow to and/or from the patient since this typically provides a control parameter for the mechanical breathing aid and may also be used in the derivation of parameters descriptive of the operation and metabolism of the lung. It is also important, particularly with respect to the measurement of expiratory gas flow, which the flow meter presents a relatively small resistance to the gas flow being measured.
Venturi-type or so-called “differential pressure” flow meters are well known in the art and are particularly suited to the clinical applications mentioned above. Such a meter generally has a tubular flow channel for conveying the gas flow to be measured and having a constriction, reducing the cross-sectional area of the flow channel and providing a resistance to gas flow. Also provided is a differential pressure gauge for measuring a pressure drop within the channel that is caused by the constriction. This pressure drop is proportional to the gas flow to be measured. For laminar flow the sensed pressure difference is directly proportional to the volume flow rate while for the more typical turbulent flow it is proportional to the square of the volume flow rate.
The venturi-type flow meter has a problem that its sensitivity and accuracy at low flow rates is relatively poor. This problem may arise particularly when the meter is employed in the measurement of gas flow to and from small children or neonates.